Photoresist technology and materials are conventionally used for forming patterns of conductive, semi-conductive, or insulating materials on a substrate in fabrication of devices such as electronic components, integrated circuits, small-scale photonic components, printed circuit boards, and interconnecting components for such devices. Using conventional photoresist materials and methods, a photoresist layer is overlaid and patterned onto a substrate or onto other patterned materials so that it either protects or exposes underlying material for an etching process that follows. The photoresist layer is normally formed of a polymeric, organic material that is substantially unaffected by a metal deposition or metal removal process and, accordingly, protects underlying areas from etching processes. A pattern is formed by imagewise exposing the photoresist material to actinic radiation through a mask or using a photographic image, such as a glass master, similar to photolithographic techniques. The exposing radiation, generally in the UV, X-ray, or electron beam domain, causes a chemical reaction in the photoresist material, affecting its relative solubility accordingly.
Photosensitive materials and compositions can be either positive-acting (that is, photosolubilizable) or negative-acting (that is, photoinsolubilizable or photocrosslinkable). Positive-working (photo) sensitive compositions are rendered soluble by actinic radiation (deep-near UV, x-ray or electron-beam) and can be removed using selective developing solutions leaving unexposed areas intact. Negative-working (photo)sensitive compositions are those which become insoluble upon exposure to actinic radiation. Selected solutions can dissolve and remove the unexposed areas of the composition while leaving the exposed portions intact. Development of such exposed materials yields negative tone images.
In general, the use of photosensitive materials for patterning is familiar to those skilled in the device fabrication arts, and years of research and development have helped to exploit photosensitive materials for a wide range of fabrication uses and to improve and refine various techniques for multi-layer device patterning using these materials.
In general, photoresist materials and techniques have been successfully applied for fabrication of a number of different types of components, however, these materials have some significant disadvantages and shortcomings. Conventional UV photoresist etching is a relatively costly process, requiring relatively high-energy radiation sources to drive the needed chemical reactions. Preparation of masks and photo-tools can be very expensive, error-prone, and time-consuming. The use of masking techniques is necessarily resolution-limiting and places considerable demands on the design of supporting optical components.
In attempts to improve upon the expense and complexity of conventional photoresist etching, a number of alternative fabrication techniques have been adapted. For example, the technology of ablation, which is the art of completely vaporizing a coating from a substrate, is a competing technology to photoresist etching. However, ablation is known to be a poor performer for complex patterning situations that require multiple layers. Repeated ablation cycles tend to degrade and debris from the impacted etching must be collected, since loose debris can be a serious problem for clean room environments.
Another competing technology to conventional photoresist employs a transfer mask. With this method, a donor sheet is used to transfer material from a substrate. The transfer mask has the advantage that loose debris is well contained. However, if the transferred material remains in contact with the substrate, tearing can occur at edges. Standoffs are sometime used to keep the transferred material from the substrate. However, when standoffs are employed loss of resolution may result and the standoffs themselves may attribute to shadowing upon the image.
Unlike photoresist substances that undergo chemical changes in reaction to light of high-energy, thermal resist materials undergo chemical or physical reactions in response to heat energy. Depending on the type of thermal resist material, the response to heat energy may take the form of ablation, or increased or decreased solubility in a particular developer, for example. In general, thermal resist materials are advantaged over photoresists by simpler chemistry, lower cost, and relative insensitivity to ambient light. Thermal resists are suitable for clean room environments where electronic circuits are manufactured.
Exemplary thermal sensitive materials and methods have been used for lithographic plate imaging, as described in International Patent Application WO 97/39894 entitled, “Heat-Sensitive Composition and Method of Making a Lithographic Printing Form with it” by Parsons et al. Thermal resist materials have also been proposed for use in electronic component manufacture, as disclosed in U.S. Pat. No. 6,423,456 entitled, “Method for Manufacture of Electronic Parts” to Kitson et al. and in U.S. Pat. No. 6,352,814 entitled, “Method of Forming a Desired Pattern” to McCullough et al.
Recent development of powerful, yet inexpensive infrared (IR) lasers has made them increasingly attractive for use in patterning by maskless lithography methods that use thermal resist materials. IR lasers provide a thermal solution that is advantaged over the heretofore-conventional photoresist etching. The use of lower cost IR laser sources with thermal resist materials in maskless lithography offers promise for dramatically reducing the cost and complexity of device fabrication as compared to conventional UV sources with photoresist materials. Maskless lithography is particularly advantaged over conventional mask-based techniques, especially with flexible substrates, large substrates, or surfaces that may not be perfectly planar.
However, a number of practical considerations remain. For example, while some inherent advantages over photoresist may be acknowledged by those skilled in the fabrication arts, further improvements in cost and performance would be needed to overcome reluctance to change from conventional photoresists to thermal resist materials and methods that are not as well known and to motivate fabricators to adapt their working methods to this alternative solution.